A common application of pressure-sensitive adhesives in the industry today is in the manufacturing of various displays, such as computer monitors, TVs, cell phones and small displays (in cars, appliances, wearables, electronic equipment, etc.). Flexible electronic displays, where the display can be bent freely without cracking or breaking, is a rapidly emerging technology area for making electronic devices using, for example, flexible plastic substrates. This technology allows integration of electronic functionality into non-planar objects, conformity to desired design, and flexibility during use that can give rise to a multitude of new applications.
With the emergence of flexible electronic displays, there is an increasing demand for adhesives, and particularly for optically clear adhesives (OCA), to serve as an assembly layer or gap filling layer between an outer cover lens or sheet (based on glass, PET, PC, PMMA, polyimide, PEN, cyclic olefin copolymer, etc.) and an underlying display module of electronic display assemblies. The presence of the OCA improves the performance of the display by increasing brightness and contrast, while also providing structural support to the assembly. In a flexible assembly, the OCA will also serve at the assembly layer, which in addition to the typical OCA functions, may also absorb most of the folding induced stress to prevent damage to the fragile components of the display panel and protect the electronic components from breaking under the stress of folding. The OCA layer may also be used to position and retain the neutral bending axis at or at least near the fragile components of the display, such as for example the barrier layers, the driving electrodes, or the thin film transistors of an organic light emitting display (OLED).
If used outside of the viewing area of a display or photo-active area of a photovoltaic assembly, it is not necessary that the flexible assembly layer is optically clear. Indeed, such material may still be useful for example as a sealant at the periphery of the assembly to allow movement of the substrates while maintaining sufficient adhesion to seal the device.
Typical OCAs are visco-elastic in nature and are meant to provide durability under a range of environmental exposure conditions and high frequency loading. In such cases, a high level of adhesion and some balance of visco-elastic property is maintained to achieve good pressure-sensitive behavior and incorporate damping properties in the OCA. However, these properties are not fully sufficient to enable foldable or durable displays.
Due to the significantly different mechanical requirements for flexible display assemblies, there is a need to develop novel adhesives for application in this new technology area. Along with conventional performance attributes, such as optical clarity, adhesion, and durability, these OCAs need to meet a new challenging set of requirements such as bendability and recoverability without defects and delamination.